A method for obtaining a height profile (or 3D topography) of an object or sample, more precisely of a surface of the object or sample, is known as Moiré 3D phase shift image processing or phase profilometry. A pattern is projected on the surface of interest. Depending on the height profile of the surface, these lines as projected on the surface are deformed with respect to the shape they would have if projected onto a plane, i.e. onto a surface of constant height. From images of the deformed lines, the height profile of the surface can be inferred. To this end, usually more than one pattern is used. For larger surfaces, the projected pattern is projected repeatedly on different areas of the surface in a stepped fashion, in order to cover the entire surface of interest.
The published US patent application US 2008/0117438 A1 discloses a phase profilometry system determining the height of a given point with respect to a reference plane using the projection of at least two distinct patterns, which are projected one after the other. The arrangement of the projection and detection devices is such that the detection does not occur in the direction of specular reflection, and therefore the surface must exhibit a diffuse reflection component of sufficient strength to produce a suitably high contrast in the resulting images used for height evaluation. Purely specular surfaces cannot be analyzed with this setup. The published PCT application WO 2006/039796 A1 overcomes this latter deficiency by adding a second detection device in order to detect light from the surface along the direction of specular reflection.
The published PCT application WO 0171279 A1 addresses the problem that the projection of multiple patterns, one after the other, is time-consuming, which is a serious disadvantage if high through-put of the analysis is paramount. The proposed solution is to project multiple patterns onto the surface of the object simultaneously, however at different wavelengths. Applying corresponding filters, images of the multiple projected patterns, deformed according to the height profile of the surface, can be obtained at once.
Restriction to a possibly weak diffusive component of scattered light obviously is a disadvantage. Including a second detection device makes the resulting apparatus more complex, and brings about additional issues like calibration and relative alignment of the two detection devices. Using projections at multiple wavelengths also makes the setup more complicated and implies possible alignment or calibration issues. There may furthermore be problems if the surface to be measured reflects some of the wavelengths employed better than others. The stepping approach, if larger surfaces are to be scanned, may lead to alignment errors of the fields of view used at each step, and therefore produce inaccuracies in the final results of the measurements.